With modern disk drives, there is a large performance (approx. 100:1 ratio) difference between issuing Input/Output (I/O) requests in a sequential order (with respect to physical location) on disk versus issuing I/O requests in a random order. The large difference in performance is predominately the result of two sources of latency, namely seek latency and rotational delay. Seek latency corresponds to the time required for the disk drive to move a physical read/write head to the location on the disk (i.e., the area on a platter in the disk drive) and the time required to allow for fine-tuning the exact position of the read/write head (commonly referred to as “head settling”). Rotational delay occurs when the read/write head is in the proper position, but the disk drive must wait for the desired sector to rotate underneath the read/write head.
The aforementioned latencies are typically measured in milliseconds. This is a very large amount of time when compared to the time increment used to quantify processing power of processors (e.g., nanoseconds, etc.), and these delays tend to dictate the performance available to a file system given an underlying disk drive.
Modern disk drives have two common mechanisms that help to minimize the aforementioned sources of latency: tagged queuing and time-of-flight I/O scheduling. Tagged queuing allows the disk drive to accept multiple outstanding I/O requests that the disk drive may then service concurrently in any order that the disk drive chooses. Time-of-flight I/O scheduling allows the disk drive to use detailed knowledge about the disk drive geometry and other physical characteristics to potentially service other I/O requests while waiting out the rotational delay for a desired block.
For example, consider a disk drive (implementing the aforementioned mechanisms) has just completed servicing an I/O request for block 1000. The next I/O request that needs to be serviced is block 1500, which is located on the same track as block 1000, but 270 degrees of rotation away. While the disk drive is waiting for the disk to rotate around to block 1500, the disk drive analyzes the other outstanding I/O requests and determines that blocks 250 and 2750, which are located on adjacent tracks, may be serviced while still allowing time for the read/write head to seek back to the track for block 1500 in time to read the block 1500 as it passes under the read/write head.